1. Field of the Invention
The present invention relates to a motion transmitting mechanism for converting a rotational motion to a linear motion. More particularly, the present invention relates to a screw-nut type driving mechanism wherein a rotation of a screw shaft is converted into a linear motion of a nut which is threadably engaged with the screw shaft.
2. Description of the Prior Art
A conventional screw-nut type mechanism includes an externally threaded screw shaft and an internally threaded driven nut which is engaged with the screw shaft. The screw shaft is adapted to be rotated by means of a suitable power source. For the purpose, the screw shaft may be formed with a worm wheel which may be engaged with a worm shaft rotationally driven by a power source. The driven nut is secured to a structural member which is to be driven in a linear direction. Since the driven member is constrained against rotation, a rotation of the screw shaft produces a linear motion of the driven nut and hence causes a linear motion of the driven member.
In a typical example of such screw-nut type driving mechanism, the driven nut is connected to the driven member by fitting the nut to a hole formed in the driven member. For this purpose, the driven nut is formed with an external surface of a polygonal cross-section with the external surface tapered in the axial direction. The hole of the driven member is correspondingly shaped so that the driven nut can be fitted to the hole. The driven nut has an externally threaded portion at the small diameter end which is adapted to project from the driven member and engage a tightening nut so that the driven nut is securely connected to the driven member. In order to provide the driven nut with a radial resiliency or flexibility, the driven nut is formed with radial slits that extend in the longitudinal direction of a portion of the nut so that the nut is divided into a plurality of radially flexible segments.
It has been found, however that, this type of screw-nut driving mechanism is disadvantageous in that, when the tightening nut is applied too strongly, the segments of the driven nut is forced toward the screw shaft with an excessive force increasing the resistance against the rotation of the screw shaft. A further problem in this type of mechanism is that the tapered external surface of the driven nut is rapidly worn or deformed since the driving force is transmitted from the nut to the driven member through the tapered surface of the driven nut. This is particularly true in case where the driven nut is made of a plastic material. Such deformation in the surface of the driven nut causes a play between the driven nut and the driven member.